1. Field of the Invention
This invention relates to marsh T-couplers especially adapted for use with seismic leader cables laid at the bottom of swampy terrains while conducting seismic prospecting.
2. Description of the Prior Art
A march "string" generally consists of a "thru" leader cable which makes the necessary electrical interconnections between spaced-apart "drop" leader cables. Each drop leader has at the end thereof one or more seismic detectors. The thru leader which is used to connect together the seismic detectors, i.e., the geophones and/or hydrophones, is generally a three-conductor cable and stretches the entire length of the marsh string, say 50 to 300 feet. The three-conductors are required to interconnect the drop leader's seismic detectors in series and/or parallel combinations, or to interconnect them in series on a marsh string that is "double ended", i.e., which has a connector at each end so that it can be connected into the recording instrument's "spread cable" from either end of the marsh string.
A "spread cable" usually consists of more than 50 wire pairs. Each wire pair collects the seismic data gathered by the land strings and/or marsh strings and transmits the collected data to the recording instruments. In a spread cable, a twisted pair of wires is employed for each separate channel of seismic data to be recorded. A seismic crew typically employs 48 or more separate seismic data channels.
In recent years, some spread cables have been replaced with telemetry systems, wherein the components of the recording system are dispersed over the line along which the seismic exploration is being conducted. A double-ended marsh string is then connected either to the spread cable of the recording system, or to the remote data acquisition units (RDAU) of the telemetry system.
Three-conductor thru leaders are primarily used: (1) when the individual marsh detectors, i.e., geophones and/or hydrophones, are to be connected in series-parallel combinations, or (2) when the detectors are to be connected in series and the string is to be double ended.
On the other hand, if all the marsh detectors are to be connected only in series, then a two-conductor thru leader can be used, and the marsh string can be single ended because it needs to be connected to the recording system only from one point.
It is possible to fabricate a double-ended marsh string utilizing only two conductors in the thru leaders, but only when the marsh detectors are connected in parallel. However, this condition is not often encountered.
Since marsh detectors are only two terminal devices, no more than two conductors are needed in a drop leader cable to electrically connect the output terminals of the detector to the conductors in the thru leader cable.
My novel marsh T-coupler now makes it possible to easily and economically fabricate marsh strings and/or to replace the seismic detectors thereon. The advantages of my invention will be better appreciated from a short review of techniques which are commonly used for fabricating a marsh T. There are three commonly used methods for fabricating a marsh T.
The first method requies that the T-connection, i.e., the interconnection between the conductors of the drop leader and the conductors of the thru leaders be totally encapsulated. This is done at the factory by molding a complete cover of neoprene or urethane over the T-connection. Unfortunately, in the harsh environment in which marsh strings are used, both the thru leaders and drop leaders often become cut. When that happens, electrical leakage occurs between the leaders' conductors and ground. Since a molded T-connection forms an integral and permanent part of the marsh string, it is difficult to open in the field the T-coupler for replacement of the particular leader cable which is cut. Hence, adequate servicing of such marsh strings in the field is virtually impossible.
The second method involves the use of two halves of a T-casing which is usually made of hard plastic. The interconnection between the conductors of the thru leader and the drop leader are made within one half of the T to which is then secured to the other half of the T. The void left inside the T-casing is then filled with a potting compound. This method has the advantage of being much cheaper than the first method. But, field servicing is still difficult because the employed potting compound strongly adheres to the inner walls of the T-casing. Consequently, the field crew finds it difficult to take apart such a T-casing, then to make the necessary interconnections, and finally to properly and completely fill the void in the T-casing with a potting compound. In an improperly potted T-casing, packets of voids remain which will tend to allow moisture penetration leading to electric leakage between the leaders' conductors and ground.
Finally, the third method for making a marsh T is described in U.S. Pat. No. 3,956,575. The patented marsh T has a cylindrical casing which defines a chamber and a single entry port. Solder balls on a disc inside the chamber anchor the bare ends of the interconnected conductors. Solder balls require soldering irons and consume time of skilled technicians who usually are not available to a seismic crew. To waterproof the chamber in the casing, its single entry port is sealed with a grommet made of a resilient material. This grommet has three axial bores for accepting therethrough the two thru leaders and the single drop leader which are to be interconnected. Each leader has a flexible external protective jacket. A cap is threadedly connected to the open end of the casing to compress and to hold the resilient grommet in its sealing position.
Under field use, frequent pulling on the drop leaders is often unavoidable, for example, in order to extract a geophone at the end of a drop leader which has been planted deep into the marsh. Such frequent pulling eventually causes the diameters of the outer jackets of the pulled drop leaders to decrease. The reduced-diameter jackets will tend to slide out from their respective sealing grommets. When that happens, moisture starts penetrating inside the chamber of the T-casing from which the jacket was pulled out. Since water is a better conductor of electricity than air is over dry earth, such moisture penetration into the chamber, through the bore in the sealing grommet from which the cable jacket was pulled out, will eventually short circuit, inside the leaky chamber, the interconnected conductors to each other and/or to ground. As a result, the desired seismic signals generated by the geophones and/or hydrophones will become short-circuited to ground giving rise to seismic map sections which will be difficult, if not impossible, to interpret.
There has been a need, therefore, for the seismic leader cables to become interconnected in a completely moisture-proof manner, even when the drop leaders are frequently pulled upon. While the marsh T described in U.S. Pat. No. 3,956,575 requires no potting and can be opened easily for testing or repair, nevertheless, field repair and installation are still relatively difficult because the patented T-coupler requires stripping the insulation from the ends of the conductors, arranging the conductors in their proper sequence, interconnecting the bare ends of the conductors, and then soldering balls onto the interconnected bare ends, all of which require time, skilled laber, and are difficult to execute in the field.
My invention will eliminate the above described and other known problems and shortcomings of the prior art techniques used to manufacture marsh T-couplers, to install marsh geophones and/or hydrophones on drop leader cables, and to assemble them into marsh strings.
A most important advantage of my invention is that it allows the component parts of a marsh string to be prefabricated. Once prefabricated, the electrical interconnections between the thru leaders and drop leaders can be made by means of polarized plug-in connectors without the use of soldering irons. Plug-in connections can be made quickly and easily by simply screwing on a cap which secures the mating connector parts together. Thus, field testing and replacement of drop leaders and their detectors is possible with my invention without using soldering irons, special tools, and special skills.
My invention further provides a marsh T-casing having three entry ports, each having a separate sealing grommet, whereas in the marsh T, described in U.S. Pat. No. 3,956,575, a single entry port is used having a single grommet for the porpose of sealing off all three leader cable entries into the marsh T. My invention uses a separate anchor for each leader jacket which prevents the above-described, jacket-diameter-reduction problem and the ensuing loss of imperviousness.
Prior to my invention, the need to completely waterproof the marsh T on one hand, and the need for rapidly connecting and disconnecting the leader cables to the marsh T on the other hand, appeared to be mutually inconsistent. Whereas the prior art requires solder balls and operator skill to accomplish the proper electrical interconnections between the conductors of the thru leaders and the drop leaders, in my invention the proper electrical connections are molded into the marsh T itself. In assembling the thru leaders and drop leaders together with the marsh T, uniquely polarized plug-in connectors are used. In this manner, human errors are completely eliminated.
In the field it is often required to change the lengths of the thru and drop leaders because the intervals between the geophones and/or hydrophones are determined by the lengths of the thru leaders between consecutive marsh T-couplers. An additional advantage of my invention lies in the flexibility it provides for changing the interval lengths between the marsh detectors, and for changing the lengths of the leader cables themselves. In the prior art, changes involving thru leader or drop leader length could only be accomplished by first rebuilding the marsh T-couplers involved, and then reassembling the entire marsh string. My invention makes it easy to change the lengths of the thru leaders and/or drop leaders. To change a leader, it is only necessary to unscrew the cap, remove the leader-connector to be changed, replace it with the desired length leader-connector and screw the cap back onto the T-casing.
Also, in the field defective marsh geophones and/or hydrophones must be replaced from time to time. My invention allows the drop leader with the defective detector to be simply and easily removed from the marsh T-casing and then to be replaced with a drop leader that has at one one my plug in connector and at the opposite end thereof a good geophone or hydrophone.
Thus, it may be fairly stated that while several types of marsh T-couplers for joining leader cables are already known and used, waterproofing of the known marsh T-couplers is rather difficult, the jackets of the leader cables are not sufficiently anchored, and quick connect/disconnect of the leader cables is not possible.
Accordingly, my invention provides a new and improved seismic marsh T-coupler which meets a number of rather specific requirements, specifically to interconnect/disconnect the leaders without the need for soldered connections, and to easily interchange different length leaders. The leaders according to my invention remain interconnected within the T-casing in a sufficiently moisture-proof and disconnection-resistant manner because the leaders' outer jackets become adequately anchored to the body of my T-coupler. In addition, simple mechanical polarizing means are provided for each connector to prevent accidental misconnections between the wires of the interconnected leader cables.